1. Field of the Invention
The present invention relates in general to control systems for an automotive automatic transmission, and more particularly to control systems of a type which controls the transmission in accordance with engine load parameters. More specifically, the present invention is concerned with a control system for an automatic transmission associated with an internal combustion engine which is operatble on a mixed fuel including gasoline and alcohol.
2. Description of the Prior Art
Hitherto, various types of control systems for an automotive automatic transmission have been proposed and put into practical use.
One of them is a type shown in Japanese Patent First Provisional Publication 61-257332. In this control system, a throttle valve angle and a vehicle speed, which can be typical parameters for representing the engine load, are used for making a gear change data map. Under running of the vehicle, an appropriate gear position is looked up from the data map and a suitable gear change is carried out providing the transmission with the appropriate gear position.
Some of the conventional control systems are of a type in which the line pressure (viz., the oil pressure produced by an oil pump) of the transmission is controlled in accordance with the throttle valve angle.
In general, as the parameters for the engine load, the amount of air fed to the engine is used in addition to the throttle valve angle and the vehicle speed.
Nowadays, as a substitute for gasoline, alcohol, such as methanol or the like, has been introduced as fuel for automotive engines as is discussed in, for example, Japanese Patent First Provisional Publication 56-98540. The engine disclosed by this publication can be operated on either gasoline, alcohol or mixture of them. That is, an alcohol sensor is used for detecting the alcohol concentration in fuel, and the amount of fuel fed to the engine is controlled or corrected in accordance with the detected alcohol concentration. However, if an engine of the type disclosed by 56-98540 publication and an automatic transmission of the type disclosed by 61-257332 publication are simply combined, the following undesired phenomena may occur.
As is seen from the graph of FIG. 4, the engine torque obtained under the same rotation speed is different between gasoline and alcohol. Thus, if the engine load parameters given by the engine of 56-98540 publication are simply used for controlling the transmission of 61-257332 publication, they fail to accurately control the transmission in accordance with the engine torque. That is, if the line pressure is controlled by the throttle valve angle, the intake air amount and the vehicle speed, which are parameters for representing the engine load, a concentration change of alcohol in fuel tends to cause a marked shift shock as well as a marked slippage of friction elements of the transmission, which deteriorate drivability of the vehicle as well as fuel consumption of the same.